Sprinkler systems in the northern climates must be drained or blown-out with air to clear the water to prevent freezing damage. In many cases the simplest installation provides only for allowing the irrigation system pipes and sprinklers to be cleared of water by blowing out the system using compressed air. This can be very damaging to the sprinklers which have water turbines which are normally water powered and rotate at a much slower speed with the water which is a relatively heavy incompressible fluid and does not generate the high turbine stator velocities produced when air, an expandable relatively light fluid, is expanded across the turbine stator onto the turbine blades.
The high turbine shaft velocities can heat the shaft and cause it to seize to the plastic housing material. This prevents the turbine from turning and renders it unusable in the future unless care is taken to limit the system air, blow-out time and pressures. This has proved to be one of the major causes for premature failure of gear driven sprinkler in colder climates, where sprinklers are used for only part of the year, and should last much longer than in warmer climates where they are run year round.
Devices are known for controlling the rotational speed of turbine-driven sprinklers. One such device, shown in Clark U.S. Pat. No. 5,375,768, is designed to maintain constant turbine speed despite variations of inlet water pressure. The patented sprinkler relies on a throttling device to direct part of the water to the turbine rotor, and a pressure responsive valve to divert some of the water around the turbine. This design, however, can not effectively limit rotational speed when the turbine is driven by a compressible fluid such as air, and still allow the turbine to run at a sufficiently high speed when it is driven by an incompressible fluid such as water because of the rapid expansion of the compressed air as it enters the turbine chamber.
Other turbine speed limiting mechanisms are known, but to applicant's knowledge, none of these are suitable for turbines which must run on both compressible and incompressible fluids.